A Good Group
A good group isn't one that you clear content with. A good group is where you have fun and work together. :) A good group isn't perfect, but it is good. You don't necessarily all become friends and keep in contact later. Maybe you join their guild or they join yours if one of you isn't in a guild and wants to be.
A good group can be a bunch of lowbies laughing behind their screens as you guys defeat bosses and add groups slowly and carefully. A good group can be fully geared highest level players who zip through it at high speed. A good group can be a single or multiple high levels escorting lower levels through. A good group can be a high level standing vanguard and only staying alive thanks to their lower level group members.
In the end, a good group doesn't clear content. A good group is where you had fun, you all were nice and supported each other, and parted ways happily and perhaps even fondly.
Saying all this because I had a fun group today of 3 level 20s and I was the lone 70. I could not have made it without their support as I kept the adds and boss on me to the best of my abilities. :) We all had a good time and parted ways nicely after beating the last boss. ^_^